1. Field of the Invention
The present invention relates to an automotive anti-skid control system which prevents skidding and thus provides maximum effective braking and assures steering effect on turns by optimally controlling each wheel cylinder pressure, and specifically to a system which assures minimum possible braking distance during braking even when temporarily installing a comparatively small-sized tire such as an emergency tire, called a temper tire.
2. Description of the Prior Art
As is generally known, on conventional automotive anti-skid control systems which can prevent wheel lock during braking, each wheel-brake cylinder pressure is optimally controlled by adjusting a slip ratio at the slipping less-traction wheel towards a predetermined reference slip ratio. The slip ratio at the slipping less-traction wheel which is subjected to anti-skid control, is generally derived from the ratio of the deviation between a vehicle speed and a wheel speed detected at the slipping less-traction wheel with respect to the vehicle speed. On the other hand, the predetermined reference slip ratio means a desired slip ratio advantageous to assure both a steering effect on turns and a shorter braking distance during braking. For instance, when the derived slip ratio at the slipping less-traction wheel exceeds the reference slip ratio, the brake-fluid pressure to the associated wheel-brake cylinder is reduced. In contrast, when the slip ratio becomes less than the reference slip ratio owing to the above-mentioned reduction of the wheel-brake cylinder pressure, the wheel-cylinder pressure is built up again so as to adjust the slip ratio towards the reference slip ratio. The braking force at the slipping less-traction wheel, which is subjected to anti-skid control, can be adjusted by automatically controlling the braking operation as if automatic pumping brake action is executed in cycles. In the same manner, the wheel-cylinder pressure may be controlled so that the wheel speed detected at the less-traction wheel is adjusted towards a target wheel speed which can be derived from the vehicle speed in consideration, of the desired reference slip ratio at the less-traction wheel subjected to anti-skid control. In case that a vehicle speed derived from rotational speeds of the output shaft of an automotive transmission is used as a vehicle speed necessary to calculate a slip ratio at a slipping less-traction wheel subjected to anti-skid control, such a vehicle speed derived from the rotational speeds of the transmission output shaft tends to fluctuate during braking. Thus, in recent years, on the assumption that there is no occurrence of acceleration-slip (wheel-spin) at respective drive wheels during braking (the accelerator pedal is not depressed), a vehicle speed is estimated on the basis of the wheel speeds detected at the respective road wheels which can be subjected to the anti-skid control. As set out below in more detail, the vehicle speed can be estimated on the basis of the detected wheel speeds and a longitudinal acceleration exerted on the vehicle body. For instance, a pseudo vehicle-speed generator has been taught in Japanese Patent Provisional Publication (Tokkai Heisei) No. 4-27650, which is assigned by the same assignee as the present application. During anti-skid control, this pseudo vehicle-speed generator operates to set a maximum wheel speed detected at a rise time point corresponding to a setting point of a flag representing that the anti-skid control system comes into operation, as an initial value of the vehicle speed. The pseudo vehicle-speed generator is so designed to define the sum of the maximum wheel speed defined as the initial value of the vehicle speed and the integral of a longitudinal acceleration detected by a longitudinal-acceleration sensor, as a pseudo vehicle speed. On the assumption that drive wheels would not be brought into acceleration-slip during braking and on the fundamental concept that the anti-skid control would be made in a manner so as to avoid wheel-lock at the slipping less-traction wheel being under the anti-skid control, during anti-skid control, the detected maximum wheel speed is traditionally used for the purpose of estimation of the vehicle speed required to calculate a slip ratio. For the purpose of detecting the wheel speeds of the respective road wheels, an angular velocity of each road wheel, which angular velocity can be regarded to be substantially equivalent to an angular velocity of the axle, is ordinarily used to derive a wheel speed of each road wheel, since it is difficult to detect the actual rotational speed of the respective road wheel. Actually, serrated gear teeth are formed on the outer periphery of a brake disk which is mounted inside of the wheel hub for co-rotation together therewith. Rotation of the disk causes changes in magnetic field strength of a coil of wire opposing the gear. The serrated gear teeth and the coil cooperatively construct a so-called wheel-speed sensor. The changes in magnetic field strength of the coil can be usually detected as changes in voltage applied to the coil. In general, the angular velocity of the respective axle is derived by wave-forming the changes in voltage, and then the wheel speed is obtained by multiplying the derived angular velocity of the axle by the outside radius of the usual or standard tire except the small-sized or different-diameter emergency tire.
During actual vehicle traveling, in the case that the tire is punctured unexpectedly, an emergency tire would be installed in place of the punctured usual tire. In recent years, the outside diameter of an emergency tire mounted on passenger cars is so designed as to be small-sized, as compared with the outside diameter of a usual tire which usual tire is initially installed at the respective road wheel. The wheel speed of the road wheel with the small-sized emergency tire, detected by the above-noted wheel-speed sensor, tends to be greater than the wheel speed of the road wheel with the usual tire, owing to a conventional calculation of the wheel speed based on the angular velocity of the axle. For example, in the event that the actual vehicle speed is maintained at a constant speed during straight-ahead driving, the axle of the road wheel with the small-sized emergency tire must rotate faster than the axle of the road wheel with the usual tire. Thus, the angular velocity of the road wheel with the small-sized emergency tire tends to become greater than the angular velocity of the road wheel with the usual tire. As previously noted, since the detected value of the wheel speed, serving as a control parameter, is arithmetically derived by multiplying the angular velocity of the axle by the outside radius of the usual tire, the calculated value of the wheel speed of the road wheel with the small-sized emergency tire tends to be greater than that of any other road wheel with the usual tire. Assuming that the calculated value of the wheel speed at the road wheel with the small-sized emergency tire is utilized as the maximum wheel speed necessary to derive the above-noted pseudo vehicle speed, an estimate of the vehicle speed, i.e., the derived pseudo vehicle speed tends to become greater than the actual vehicle speed.
In the anti-skid control executed in accordance with the above-noted undesiredly greater estimate of the vehicle speed, a target wheel speed, which is derived on the basis of the estimate of the vehicle speed in such a manner as to satisfy the reference slip ratio, also becomes greater than an ideal target wheel speed which will be derived on the basis of the actual vehicle speed. The target wheel speed based on the estimate of the vehicle speed is often used as a threshold value necessary to determine whether pressure-reduction of brake fluid fed to the wheel brake cylinder should be started during the anti-skid control. In case that the calculated wheel speed at the road wheel with the small-sized emergency tire is utilized as the maximum wheel speed necessary for estimation of the vehicle speed, the detected value of the wheel speed at the respective road wheel generally tends to be less, as compared with the estimated vehicle speed. In the anti-skid control executed for adjustment of a wheel speed of a road wheel with the usual tire after the brakes are applied, pressure-reduction is generally started at the time point when the detected value of the wheel speed at each road wheel with the usual tire becomes below the target wheel speed in accordance with a decrease in the detected value of the wheel speed. As set out above, since the target wheel speed based on the estimated vehicle speed tends to be set at a higher level than the target wheel speed based on the actual vehicle speed in the case that the small-sized emergency tire is installed, the pressure-reduction start timing of the anti-skid control based on the estimated vehicle speed is thus advanced as compared with the pressure-reduction start timing of the anti-skid control based on the actual vehicle speed. Alternatively, in case of the anti-skid control wherein the above-noted target wheel speed is used as a threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder which pressure build-up is executed after the pressure-reduction mode of the anti-skid control for the purpose of the recovery of the wheel speed to a direction wherein the wheel speed increases, supposing the respective detected value of the wheel speed is below the target wheel speed, pressure build-up is started at the time when the detected value of the wheel speed at each road wheel with the usual tire becomes above the target wheel speed in accordance with an increase in the detected value of the wheel speed. Since the target wheel speed based on the estimated vehicle speed tends to be set at a higher level than the target wheel speed based on the actual vehicle speed in the case that the small-sized emergency tire is installed, the pressure build-up start timing of the anti-skid control based on the estimated vehicle speed is retarded as compared with the pressure build-up start timing of the anti-skid control based on the actual vehicle speed. As set forth above, in both the pressure-reduction mode and the pressure build-up mode during anti-skid control, the wheel-cylinder pressure at the respective road wheels with the usual tire would be undesiredly set at a small value, because of retardation of the pressure build-up start timing and advancement of the pressure-reduction start timing. This results in an unexpectedly smaller braking force at the respective road wheel with the usual tire, as compared with the road wheel with the emergency tire. As a consequence, there is a possibility of increase in braking distance of the vehicle. To avoid the above-noted problem, further improved anti-skid control systems have been proposed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 2-169362 and Japanese Patent Provisional Publication (Tokkai Heisei) No. 3-67764, for example. The former publication will be hereinafter referred to as a "first prior art", while the latter publication will be hereinafter referred to as a "second prior art". The first prior-art anti-skid control system is designed such that a maximum wheel speed detected at the road wheel with the small-sized emergency tire is not used for estimation or calculation of the vehicle speed, only when satisfying a predetermined condition wherein the deviation between maximum and minimum detected values of the wheel speeds of road wheels respectively having a usual tire except a road wheel having a small-sized emergency tire is kept within a predetermined first threshold and additionally the deviation between the maximum wheel speed detected at the road wheel with the emergency tire and the second greatest wheel speed corresponding to the maximum wheel speed of the road wheels with the usual tire is maintained above a predetermined second threshold for a predetermined first period of time or more. In the first prior-art anti-skid control system, in case that the deviation between the maximum wheel speed and the minimum wheel speed is maintained below a predetermined third threshold for a predetermined second period of time, the wheel speeds detected at all road wheels, are used for estimation or calculation of the vehicle speed, that is, the anti-skid control system operates to, release inhibition of calculation of the vehicle speed based on the above-noted maximum wheel speed. According to the first prior-art anti-skid control system, the vehicle speed can be arithmetically estimated from the detected value of the wheel speed at the respective road wheel with the usual tire except the detected value of the wheel speed at the road wheel with the small-sized emergency tire, before initiation of the anti-skid control. Therefore, the estimated or calculated value of the vehicle speed would be essentially equivalent to the actual vehicle speed, except a particular case such as an acceleration slip wherein drive wheels considerably slip at their wheel speeds above the vehicle speed. Assuming that the estimate of the vehicle speed which estimate is arithmetically derived from the detected values of wheel speeds other than the wheel speed detected at the small-sized emergency tire, is utilized for the anti-skid control, an optimally controlled braking force may be provided at least for the road wheels with the usual tire by the aid of a precise adjustment of the wheel-cylinder pressure.
In the second prior-art anti-skid control system, the vehicle speed is estimated or derived from the maximum value of all wheel speeds detected at the road wheel with the emergency tire as well as at the road wheels with the usual tire, in the conventional manner. The second prior-art anti-skid control system is characterized in that a corrected vehicle speed set at a lower level than the estimated vehicle speed by a predetermined correction value is used as a threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder, and that a decision is made to determine whether a different-diameter tire such as a small-sized emergency tire having an outside diameter different from the other tires is installed, and that the above-mentioned predetermined correction value is increasingly compensated in the presence of the decision instruction representing that the tire having the different diameter is installed, so as to reduce the threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder. That is to say, in the second prior-art anti-skid control system, a decreasingly compensated vehicle speed obtained by subtracting the increasingly compensated correction value from the estimated vehicle speed, is used as the target wheel speed, only in case that the different-diameter tire is installed. Assuming that the detected value of the wheel speed of the road wheel with the different-diameter tire, corresponding to the maximum wheel speed, is greater than the detected value of the wheel speed at any other road wheel with the usual tire by a predetermined ratio, that is to say, the ratio of the outside diameter of the different-diameter tire to the usual tire is set at a predetermined constant value, and that the setting relation of the increasingly compensated correction value, for example 10 km/h, to be subtracted from the estimated vehicle speed in case of the different-diameter tire installed at either one of the road wheels with respect to the uncompensated predetermined correction value, for example 3 km/h, to be subtracted from the estimated vehicle speed in case of the usual tires installed at all road wheels, is in proper correlation with the reciprocal of the above-noted outside-diameter ratio of the different-diameter tire to the usual tire, the target wheel speed based on the maximum wheel speed can be properly compensated as if the wheel speed detected at the road wheel with the different-diameter tire can be regarded as the wheel speed detected at the road wheel having the outside diameter substantially equivalent to the usual tire, even when the vehicle speed is estimated from the maximum wheel speed corresponding to the wheel speed at the different-diameter tire such as a small-sized emergency tire. Thus, in transit from the pressure-reduction mode of the anti-skid control to the pressure build-up mode of the anti-skid control, since the target wheel speed is decreasingly compensated as set out above and additionally the wheel speed at the road wheel subjected to the anti-skid control can quickly reaches the compensated target wheel speed serving as a threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder, the pressure build-up start timing can be advanced and thus the wheel speed is smoothly recovered to the wheel-speed increasing direction. Accordingly, on the anti-skid control executed at least for the road wheels with the usual tire, the wheel-cylinder pressure may be optimally adjusted owing to the compensated target wheel speed. However, the second prior-art does not refer to the correlation between the ratio of the outside diameter of the different-diameter tire to the usual tire and the setting relation of the increasingly compensated correction value, such as 10 km/h, to be subtracted from the estimated vehicle speed in case of the different-diameter tire installed at either one of the road wheels with respect to the uncompensated predetermined correction value, such as 3 km/h, to be subtracted from the estimated vehicle speed in case of the usual tires installed at all road wheels.
In the first prior-art anti-skid control system, there is a possibility that the anti-skid control is uncertainly executed, because, until the deviation between the maximum wheel speed and the minimum wheel speed is below the predetermined threshold, the maximum wheel speed which would be detected at the road wheel with the different-diameter tire, cannot be used for estimation of the vehicle speed necessary for the anti-skid control, that is, the first greatest one of four detected wheel speeds required for estimation of the vehicle speed is neglected. For instance, in the event that the road wheels with the usual tire except the small-sized tire are all locked or beginning to lock, the deviation between the maximum wheel speed which would be detectable at the road wheel with the small-sized tire and the minimum wheel speed which would be detectable at the road wheel with the usual tire becomes great. In this case, the deviation, is thus maintained above the predetermined threshold. As previously noted, the vehicle speed is estimated from the second greatest wheel speed under a specified condition in which the maximum wheel speed is neglected. Under such a condition, the estimated vehicle speed will be set to a relatively smaller value than the actual vehicle speed. Necessarily, the target wheel speed will be also set to a comparatively small value on the basis of both the reference slip ratio and the relatively small estimate of the vehicle speed. Supposing this target wheel speed is set at a threshold value necessary for initiation of pressure-reduction at the wheel-brake cylinder, on the anti-skid control executed after braking, it is difficult that the respective detected wheel speed becomes below the target wheel speed, since the target wheel speed tends to be set at the small value in accordance with the decrease in the detected value of the wheel speed at the respective road wheel with the usual tire. As a result, the pressure-reduction start timing of the anti-skid control based on the estimated vehicle speed may be retarded as compared with the pressure-reduction start timing of the anti-skid control based on the actual vehicle speed. In contrast to the above, on the supposition that the target wheel speed is set at a threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder in transit from the pressure-reduction mode of the anti-skid control to the pressure build-up mode according to the recovery of the wheel speed to the wheel-speed increasing direction, there is a possibility that the wheel speed of the road wheel subjected to the anti-skid control can be rapidly recovered to the target wheel speed. In this case, the pressure build-up start timing of the anti-skid control based on the estimated vehicle speed tends to be advanced as compared with the pressure build-up start timing of the anti-skid control based on the actual vehicle speed. As set forth above, in both the pressure-reduction mode and the pressure build-up mode during anti-skid control, the wheel-cylinder pressure at the respective road wheels with the different-diameter tire as well as the usual tire would be undesiredly set at a great value, because of retardation of the pressure-reduction start timing and advancement of the pressure build-up start timing. This results in an unexpectedly greater braking force at the road wheel with the usual tire other than the emergency tire. As appreciated, there is a possibility that each wheel speed cannot be properly adjusted within towards an optimal wheel-speed range adequate to get the reference slip ratio.
As previously explained, in the second prior-art anti-skid control system, the detected value of the wheel speed at a road wheel with a different-diameter tire such as a small-sized emergency tire and the detected values of the wheel speed at road wheels with a usual tire are all used for the purpose of estimation or calculation of a vehicle speed utilized as a control parameter of the anti-skid control, and additionally a target wheel speed serving as a threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder is set at a decreasingly compensated speed level by increasingly compensating the above-noted predetermined correction value necessary for the estimate of the vehicle speed in the presence of the decision instruction representing that the different-diameter tire is installed. However, the anti-skid control system made according to the second prior-art does not sufficiently take account of the correlation between the ratio of the outside-diameter of the different-diameter tire to the usual tire and the setting relation of the increasingly compensated correction value, such as 10 km/h, to be subtracted from the estimated vehicle speed based on the detected wheel speeds at all road wheels including the different-diameter road wheel with respect to the uncompensated predetermined correction value, such as 3 km/h, to be subtracted from the estimated vehicle speed based on only the detected wheel speeds at the road wheels with the usual tire. As detailed in the description of the second prior art (the Japanese Patent Provisional Publication No. 3-67764), since the predetermined correction value to be subtracted from an estimate of the vehicle speed based on only the wheel speeds detected at the road wheels with the usual tire is set univocally at 3 km/h, and additionally the increasingly compensated correction value to be subtracted from an estimate of the vehicle speed based on the wheel speeds detected at one road wheel with the different-diameter tire and at the other road wheels with the usual tire is set univocally at 10 km/h, the target wheel speed which is used as a threshold value necessary for initiation of pressure build-up at the wheel-brake cylinder and calculated in consideration of either the predetermined correction value or the increasingly compensated correction value, cannot always reflect the outside-diameter ratio of the different-diameter tire to the usual tire accurately, in case that the reciprocal of the ratio of an estimate of the vehicle speed based on the maximum wheel speed corresponding to the detected value of the wheel speed at the road wheel with the different-diameter tire with respect to an estimate of the vehicle speed based on the detected values of the wheel speed at the road wheels with the usual tire, is not in proper correlation with the setting relation between the increasingly compensated correction value (e.g. 10 km/h) and the predetermined correction value (e.g. 3 km/h). In this case, if the wheel-brake cylinder pressure, i.e., the braking force is controlled in accordance with the target wheel speed derived in the manner as set out above, there is a possibility that the respective wheel speed cannot be adjusted within towards the above-noted optimal wheel-speed range adequate to satisfy the reference slip ratio with regard to the road wheels with a usual tire. Furthermore, it will be appreciated that the wheel-speed control for the road wheel with a different-diameter tire cannot be executed precisely in case that such a target wheel speed which speed will be derived in accordance with the control procedure of the second prior-art anti-skid control system, is applied for the purpose of adjustment of the wheel speed at the road wheel with the different-diameter tire.